Some beaches in Japan are made of tiny, star-shaped skeletons.

The 'star sand' found on Japanese beaches like Hoshisuna-no-hama is primarily composed of the tests, or shells, of a specific genus of foraminifera known as Baculogypsina sphaerulata. These single-celled protists live among the seagrass and algae in the shallow coral reefs surrounding the Yaeyama Islands. When the organisms die, their pointed calcium carbonate skeletons are transported by ocean currents and deposited on the shoreline.Each individual grain measures approximately 1 to 2 millimeters in diameter and features several radiating spines that give it a distinct stellar appearance. Marine biologists note that these organisms have existed for over 500 million years, making them vital indicators of environmental health and ocean acidification levels. The abundance of these shells is so great that they form entire dunes on islands like Taketomi and Iriomote.Local Okinawan folklore provides a poetic origin, suggesting these stars are the offspring of the North Star and the Southern Cross, killed by a giant sea serpent. In reality, the accumulation of these biogenic sediments is a slow process that takes thousands of years to reach the volume seen today. Researchers from the University of Tokyo have studied these deposits to understand historical changes in sea levels and reef productivity in the Pacific region.